Regularly stay by river and eat fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a type of fish)
'regularly stay by river and eat' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation).
'and' means one lot of letters go next to another.
'eat' becomes 'dine' (to dine is to eat a meal).
The alternating letters of 'stay' are 'sa'.
'sa'+'r'+'dine'='SARDINE'
(Other definitions for sardine that I've seen before include "A diner's small fish" , "Small pilchard" , "Fish (often tinned)" , "Young pilchard" , "Fish in tin?" .)